The present invention relates to laser bar code scanners using rotating polygon mirrors and more particularly to a method and apparatus for fastening a polygon mirror onto a motor that results in assembly which can be easily balanced by mass and geometric centering without special tools or secondary operations.
Laser bar code scanners, as known in the art, employ a finely focused light beam repetitively scanned across the bar code rotating a polygon mirror that deflects the source laser beam. A photodetector responds to laser light reflections of the scanned bar code. The label represents information encoded as a series of various widths formed on a contrasting background. Difference in reflectance of the bars compared to the spaces produces a modulated optical signal. The optical signal is then converted to an electrical signal by the photodetector and that signal is further processed and then decoded.
Combined irregularities of the polygon mirror geometry and drive motor rotation can produce undesirable errors in the location of the resulting scans, especially those at right angles to the location of the intended scan line. These errors are known in the field as "tracking errors" and they become particularly important and limiting when the bar code reader is pushed to operate at high rates and at exceptional scan accuracy. There are four major sources of tracking errors that may be contributed to a scanning system by a mirror/motor assembly:
1. Errors in the angular location of polygon mirror facets that are molded on or positioned about the circumference of a cylindrical support body with respect to the datum surface of the polygon body. PA1 2. Errors in the fastening of the polygon mirror body to the motor shaft. PA1 3. Errors in the rotation of the motor shaft attributable to bearings, lack of rotor stiffness, thermal gradients causing rotor warp, and dynamic imbalance. PA1 4. Errors in flatness within each mirror facet.
To compensate for these errors, efforts have focused on expensive components fabricated with very tight tolerances and/or time consuming balancing operations. This invention shows that the dominate errors, which are usually associated with source nos. 2 and 3 above, may be addressed more economically by using the unique, yet low cost hardware and method described herein below, which has proved to result in minimum tracking errors with relatively inexpensive, easily assembled low tolerance component mirror units and motors.
Some of these problems discussed above have been addressed in the prior art. Prior methods for correcting tracking errors fall into two basic categories, active and passive. Active systems involve auxiliary small-angle deflectors located in the optical system to compensate for motor/polygon-induced errors in tracking. Passive correction schemes usually utilize additional optical elements to reduce polygon-induced tracking errors.
For example, U.S. Pat. 3,750,189, Fleischer, discloses the use of cylindrical lenses in the scanning optical imaging system. These cylindrical lenses cause focusing in one direction of the parallel rays of light directed toward the polygon scan mirror. This focusing in one direction creates a line image on the facet running in the scan direction. The reflected rays from the polygon mirror are recollimated by a second cylindrical lens. The result of the one dimensional imaging is to permit scanning in one direction, namely the direction of rotation of the polygon, but it prevents scanning in the direction at right angles to it.
Another passive approach involves the use of a retroreflecting prism which first accepts the first scan beam from a polygon facet and returns those rays to the same facet. The second reflection will have an identical error but with the reverse sign of the rays reflected in the first bounce. The result is a cancellation of the facet error.
These prior techniques have added complexities and costs, and exhibit operating and/or servicing deficiencies in the overall performance of the scanner.
Thus it is an aspect of the invention to provide a systematic method for assembly and error compensation of polygon motor assemblies, and to provide a unique yet inexpensive set of mounting components and readily available fasteners to carry out the method.
It is a further aspect of the invention to provide a mounting scheme and balancing technique by which a scan line with a minimum tracking error is achieved without utilizing additional active or passive optical compensating systems, or time consuming secondary balancing operations.